J. Phys. Chem. A 2003, 107, 19-24
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Formation of Molecular Iodine from the Two-Photon Dissociation of CI4 and CHI3: An Experimental and Computational Study Eric D. Tweeten, Benjamin J. Petro, and Robert W. Quandt* Department of Chemistry, Illinois State UniVersity, Normal, Illinois 61790-4160 ReceiVed: July 15, 2002; In Final Form: October 22, 2002
The formation of electronically excited molecular iodine from the two-photon photodissociation of CI4 and CHI3 was investigated using dispersed fluorescence and ab initio calculations. Molecular iodine was formed in the D, D′, and E ion-pair states from both CI4 and CHI3. In the photodissociation of CHI3, the intensity of the D′ band is decreased and the E band is increased relative to those from CI4. This intensity shift is explained in terms of the energetics of the carbene photofragments. Intrinsic reaction coordinate calculations were performed at the MP2 level of theory using the LANL2DZ basis set to characterize the dissociation pathways for CI4 and CHI3. The results of the calculations show the presence of three transition states and an ion-pair isomer intermediate for both molecules. The structure of the transition states for the formation of molecular iodine is in agreement with the asynchronous concerted dissociation mechanism proposed by Dantus and co-workers for CX2Y2 halocarbons.
I. Introduction The photodissociation of halocarbons has received much interest due to the role these species play in stratospheric ozone depletion and as greenhouse gases.1 The formation of atomic halogens, in both the 2P1/2 and 2P3/2 spin states, upon excitation to low-lying energy states of halocarbon parent molecules has been extensively studied.2 However, in recent years, the formation of electronically excited molecular halogens from multiphoton dissociation of halocarbons has received renewed interest. In a series of papers, Dantus and co-workers studied the femtosecond photodissociation dynamics of several gem-dihaloalkanes3,4 and other mixed dihaloalkanes.5 In their work, they found that with 96 154 cm-1of excitation energy the molecular iodine photoproduct was formed primarily in the D′ ion-pair state. They also found that the dissociation was fast (τ < 50 fs) and proceeded through an asynchronous concerted mechanism. Farmanara et al. found similar results in their studies of CF2I2.6 While several groups have looked at the photodissociation of CX2Y2 halocarbons, where X ) H or F and Y ) Cl, Br, or I, little or no work has been done on CXY3 or CY4 molecules. In this work, we report the formation of highly excited molecular iodine from the two-photon photodissociation of both CI4 and CHI3. The photodissociations were studied using both dispersed fluorescence and ab intio computational methods. II. Experimental Section The experimental technique used consists of flowing ∼50250 mTorr of CI4 (Aldrich Chemicals), CHI3 (Eastman), or, for comparative purposes, neat I2 (Mallinckrodt) through an 8 cm cubic cell made of stainless steel with five fused silica windows. The back window of the cell was offset by an additional 8 cm with an aluminum tube to reduce scatter. The 193 nm photolysis source was an ArF excimer laser (Lambda Physik, Compex 110) operating at 10 Hz and 100 mJ. The excimer beam was focused into the cell with a quartz lens (f ) 120 mm) with great care being taken to ensure that the focal point was in the aluminum
tube to prevent the absorption of three or more photons in the probe region. It should be noted that without the quartz lens no fluorescence was observed. With the quartz lens in place, intense fluorescence signals were seen for both CHI3 and CI4 and were attributed to photoproduct emission. This fluorescence was collected at right angles to the excimer beam with either a photomultiplier tube (Electron Tubes Limited model 9129b) for the time and power dependence studies or a collimating lens coupled to a fiber optic cable for the dispersed fluorescence work. The fiber optic then transmitted the collected light to an asymmetric crossed Czerny-Turner monochromator (Ocean Optics model S2000). An effective slit width of 8 mm and a 600 lines/mm grating give the monochromator a resolution of 1 nm. The dispersed light was detected with a CCD array, and the resulting signal was stored on a personal computer for later analysis. All chemicals, with the exception of CI4, were used without further purification. The CI4 was washed with a saturated sodium thiosulfate and water solution to remove any residual I2. All chemicals were stored in opaque containers, and in a freezer when not in use, to minimize photodegradation. III. Computational Section The Gaussian 987 electronic structure package was utilized on either a Linux-based personal computer or a Silicon Graphics O2 workstation for all calculations. To account for relativistic effects, all of the calculations employed the LANL2DZ basis, which consists of the Los Alamos effective core potential plus double-ζ valence-only basis for iodine.8-10 For first row atoms, the D95 double-ζ basis was used, leading to 41 basis functions for CI4 (35 for CHI3).11 Geometries and transition-state structures were optimized at the MP2 level of theory. Convergence criteria for the geometry optimizations were that the rms gradient was less than or equal to 3 × 10-4 and the maximum component of the gradient was less than or equal to 1.2 × 10-3. Single-point energies, at the MP4 level of theory using the same basis set, were then determined for the optimized structures. Neither the MP2 nor MP4 values were corrected for vibrational zero-point energies.
10.1021/jp021610j CCC: $25.00 © 2003 American Chemical Society Published on Web 11/27/2002
20 J. Phys. Chem. A, Vol. 107, No. 1, 2003
Figure 1. Dispersed photoproduct fluorescence spectra from the 2 × 193 nm photodissociation of CI4. The data shown are an average of several spectra and were corrected by subtracting an averaged background signal that was obtained in the absence of CI4.
Tweeten et al.
Figure 2. Dispersed photoproduct fluorescence spectra from the 2 × 193 nm photodissociation of CHI3. The data shown are an average of several spectra and were corrected by subtracting an averaged background signal that was obtained in the absence of CHI3.
All transition states were confirmed by the presence of a single imaginary frequency in the vibrational analysis, and intrinsic reaction coordinate (IRC) calculations were run at the MP2 level of theory to verify that they corresponded to the correct reactants and products. In addition, a natural bond orbital (NBO) analysis12 was carried out for all reactants, products, and transition states, once again at the MP2 level of theory, to determine the charge distribution using natural population analysis.13,14 IV. Results and Discussion A. Power Dependence. Upon excitation with focused 193 nm light, both CI4 and CHI3 exhibit strong UV and visible photoproduct fluorescence; however, upon removal of the focusing lens, this fluorescence disappears. To determine the number of photons involved in the photodissociation, the photoproduct fluorescence intensity as a function photolysis laser power was measured. A least-squares fit of the data gave power dependence values of 1.59 ( 0.17 for CI4 and 1.80 ( 0.20 for CHI3 indicating a two-photon process in each molecule. This is consistent with the results of Bersohn and co-workers for diiodomethane and iodoform.15 They found that there is a barrier to the formation of molecular iodine from low-energy states (
